Modeling the influence of macro‐roughness on gravel‐bed river morphodynamics

Abstract

Gravel‐bed rivers adjust their shape in response to environmental changes, including the addition or removal of macro‐roughness like logs and boulders. River restoration often involves adding macro‐roughness to reduce flow velocity and increase channel‐floodplain connectivity, but our understanding of channel response to changes in macro‐roughness is limited, in part because of a lack of theory that links macro‐roughness to reach‐scale channel evolution. We ask how macro‐roughness affects gravel‐bed river geometry, and how rivers respond to step changes in macro‐roughness as might occur during restoration. We develop a process‐based model for gravel‐bed river evolution that accounts for macro‐roughness effects, and explore modeled equilibrium channel form as well as transient response to macro‐roughness changes. We also apply the model to a case study from Washington State, USA, where macro‐roughness primarily comes from wood loading. Equilibrium simulations reveal three regimes of behavior as roughness increases: one in which modest macro‐roughness is predominantly accommodated via bank erosion and widening, one in which greater roughness is accommodated by coupled widening and aggradation/steepening, and one in which overbank flow triggers rapid aggradation that fills the channel. Trajectories of channel response to roughening can be nonmonotonic and nonlinear, implying that perceived effects of roughness changes may differ based on which measurements are made and when. The field case study suggests that the model can be parameterized using data from natural systems, and may, despite inherent simplifications, be useful for conceptualizing how macro‐roughness influences the evolution of gravel‐bed river shape.

Citation

Shobe, Charles M.; Scott, Daniel N. 2026. Modeling the influence of macro‐roughness on gravel‐bed river morphodynamics. Water Resources Research. 62: e2026WR044209.